Project Summary I am a physician-scientist determined to improve our understanding and treatment of mood and anxiety disorders. This career development award will support me to study the function of serotonin during emotional behaviors while also acquiring the training I need to successfully transition to independence. Serotonergic axons travel great distances to diverse targets throughout the brain. Many targets are critical for emotional behaviors, but the logic by which serotonin modulates these varied targets is unknown. Determining this logic is essential for understanding serotonin?s diverse functions. It is also vital to human health, as first line antidepressants globally increase serotonin in all targets. Do serotonergic neurons normally speak with one voice, releasing serotonin into their diverse targets in unison? Or are subgroups of serotonergic projections recruited to play specific roles during specific emotional behaviors? Answering this question is crucial to understanding the role of serotonin in emotional behaviors. We discovered that serotonergic activity, measured globally, is increased both during rewarding behaviors and during anxiety-like behaviors. We hypothesize that these behaviors activate subpopulations of serotonergic neurons with distinct projections. Testing this hypothesis will fill a critical gap in the field. To fill this gap, I will complete three aims. In Aim 1, I will image serotonergic neurons in the dorsal raphe nucleus with single cell resolution during both rewarding and anxiety-like behaviors. I will determine if subpopulations of serotonergic neurons encode different types of emotional stimuli. I will further specify the logic of serotonergic activity by independently imaging different serotonergic projections during emotional behaviors. I will thereby determine if the projections work in concert or if they are recruited by different behaviors. In Aim 2, I will use projection specific optogenetic silencing to determine the functional role of the activity we discover with imaging. In Aim 3, I will determine how normal patterns of serotonergic activity change after stress, a major risk factor for depression and anxiety. I will conduct this work within the New York State Psychiatric Institute and Columbia University, a leading center for psychiatric research and treatment. Under the mentorship of Dr. Ren Hen, an authority on serotonin with a proven track record mentoring early career scientists, I will complete four training aims. First, I will develop expertise in the role of serotonin in emotional behavior. Second, I will become an expert applying statistical approaches to analyzing the firing properties of individual neurons during behavior. Third, I will learn to use optogenetics to manipulate neural activity during behavior. Fourth, I will become experienced using models of chronic stress. Upon completing these aims, I will be on solid footing transitioning to independence.